Castings of said brass are used in valves, water pipes and pipe connections of water installations etc., whereby dezincification resistance is one of the most important requirements of the material. This property is tested with an accelerated dezincification test according to the standard SFS 3873.
In practice the structure has been found dezincification resistant, when the .alpha.-phase of brass is inhibited, e.g. with arsenic, to be dezincification resistant, and the .beta.-phase in the structure susceptible to dezincification is uniformly distributed in the structure. The effective Cu percentage of brass with a given cooling rate influences in the first place the decomposition of the .beta.-phase.
The effective Cu percentage has also a decisive effect on the castability of the alloy (fluidity, surface checkings and shrinkage). When realizing good castability and dezincification resistance the effective Cu percentage can only vary within very narrow limits.
In order to obtain said properties, chill casting processes use ingots intended for dezincification resistant castings and return scrap from said process as raw material for the melt.
In practice zinc volatilizes from the melt in a continuous melting process, and often a part of the aluminum functioning as an alloying element is oxidized. Hereby the effective Cu percentage of the melt increases. An abundant use of return scrap has also an effect, because the Zn and Al percentages of the scrap have decreased in earlier meltings. Thus, without analyzers the foundries have difficulties in knowing the composition of the process melt and in making analysis corrections.
Many foundries do not have analyzers with which the chemical composition of the melt could be determined. On the other hand, the analysis processes in use are slow and the analysis accuracy obtained with these is not always sufficient in view of calculating the effective Cu percentage. Especially when the errors in the percentages of different elements cumulate, the error in the effective Cu percentage can be of the same order as the allowed variation. With the analysis processes in use, the analysis correction directions of the melt cannot either be automatically obtained. As a drawback of the determination of the effective Cu percentage based on the elementary analysis can also be considered that there is not fully an unanimity about the coefficients (Zn equivalents for the different elements) in the calculation formula for the effective Cu percentage.
One important advantage of the invention can be is that the effective Cu percentage of the melt can be determined automatically and quickly, and simultaneously the alloying element calculated according to the method for correcting the composition of the melt can be made so that the cast properties are optimal and the castings are dezincification resistant. It must be particularly stressed that the analysis with its possible correction directions is obtained before using the melt. The analysis is also clearly more trustworthy than what can be obtained with the processes in use.